The diffusion of biopolymers is important in the dynamics of a living cell, the assembly of a virus and the purification of biomedical materials by column chromatography. Most systematic studies of biopolymer diffusion have been made on very dilute solutions. In contrast, macromolecule species in vivo may be present at concentrations which are high enough that physical interactions between neighboring macromolecules may affect their behaviors and solution properties. Quasi-elastic light scattering spectroscopy (QELSS) is broadly used to measure the diffusion coefficient of dilute macroparticle suspensions. Its application to more complex, concentrated solutions is obstructed by the difficulty in interpreting QELSS spectra in such cases. Our objectives in this proposal are twofold. First, develop methods for interpreting QELSS spectra of concentrated macromolecular mixtures and second, to develop a body of experience on the behavior of biopolymers diffusing through a macroparticulate background. Our major approach is based on the use of optical probes: materials labelled to scatter far more light than the background macroparticle suspension (chosen to be isorefractive with the solvent) through which the probes move. The use of optical probes greatly simplifies the interpretation of the spectrum. Optical labelling has been demonstrated experimentally, using polystyrene latex as a probe and water:poly-acrylic acid, water:polyethylene oxide, and water:serum albumin as background solutions. If the probe species is dilute, its scattering spectrum is determined by its tracer (single-particle) diffusion coefficient, the other solutes having only a secondary effect on the spectrum.